Analytical Studies on Ductile Fracture Behavior Near the Crack-Tip

本研究的目的是对韧性材料裂纹尖端的变形和断裂行为进行解析研究，并与实验结果进行比较，提出裂纹尖端延性断裂的判据。采用Gurson本构方程模拟微孔洞的形核和长大，采用大变形有限元分析方法研究了开孔和复合型载荷作用下缺口和裂纹尖端附近的变形和微孔洞集中特性。为了考察其对裂纹尖端变形和断裂行为的影响，还假定在裂纹尖端附近存在一个大孔洞。结果表明，在复合型载荷作用下，裂纹尖端一角锐化，另一角钝化，随着II型分量的增加，应变和孔洞体积分数集中在尖角处。钝化角附近的宏观孔洞迅速长大，并表现出强烈的相互作用，微孔洞分数集中在宏空洞和钝化裂纹拐角之间的位置，而锐化角点附近的宏空洞对裂尖变形和断裂性能影响不大。提出了广义裂纹尖端张开位移等参数作为复合型载荷下控制裂纹萌生和扩展的参数，并与实验结果进行了比较。在这些参数中，临界含气率、剪切局部化条件、裂纹尖端塑性功和广义裂纹尖端张开位移被认为是延性断裂准则的候选参数。还需要进一步的调查才能得出明确的结论。

It is the purpose of the present research program to investigate analytically the deformation and fracture behavior at the crack-tip of tough-materials, to make a comparison with the experimental results, and to propose a criterion for ductile fancture at the crack-tip. Large deformation finite element analyses are performed to study deformation and microvoid concentration properties near a notch or a crack-tip under opening or mixed mode loadings with Gurson's constitutive equation used for simulating nucleation and growth of micrvoids. A macrovoid is also assumed to be located near the crack-tip in order to examine its effect on deformation and fracture behavior of crack-tip. It is shown that under mixed mode loadings one corner of a crack-tip sharpens while the other corner blunts, and the strain and void volume fraction become concentrated at the sharpened corner with the increase of mode II component. A macrovoid near the blunted corner grows rapidly and a strong interaction is evidenced with microvoid fraction concentrated at the location between the macrovoid and blunted crack corner whereas a macrovoid near the sharpened cormer is found to have little effect on crack-tip deformation and fracture properties. A generalized crack-tip opening displacement and other quantities are proposed as parameters that control onset and growth of a crack under mixed mode loadings and are compared with the experimental results. Among these parameters, the critical void fraction, the condition of shear localization, the plastic work at the crack-tip, and the generalized crack-tip opening displacement are thought to be candidates for ductile fracture criterion. Further investigations will be required bofore a definite conclusion can be drawn.